At the time, the future cardinal also edited the official publication of the documents from the Joint Synod of the Dioceses in the Federal Republic of Germany for the 1971-1975 Synod of Würzburg.
In June 1983, he was appointed Bishop of Mainz and received episcopal ordination that October. A year later, he became a member of the circle for dialogue between representatives of the German Bishops Conference and those from the Council of the Evangelical Church in Germany.
From March 1986-1988 he also became a member and later president of the Lutheran-Catholic dialogue between the World Lutheran Federation and the Pontifical Council for the Promotion of Christian Unity.
The two entities in 2017 signed their latest joint-statement to mark their shared commemoration of the 500th anniversary of the Protestant Reformation. In a major ecumenical move, Pope Francis traveled to Lund, Sweden from Oct. 31-Nov. 1, 2017, for a joint-commemoration of the landmark anniversary.
In addition to his role in helping advance ecumenism, Cardinal Lehmann was elected president of the German Bishops Conference in 1987 – a position he held for 20 years, being reconfirmed in the role in 1993, 1999 and again in 2005.
In 1995 he was named Special Secretary of the First Special Assembly for Europe of the Synod of Bishops, and from 1988-1998 was a member of the Congregation for the Doctrine of the Faith.
The prelate received awards from universities and institutions all over the world, and was the author of several publications.
He participated in the conclave of 2005 that elected Benedict XVI as Pope, as well as the conclave of 2013 that elected Pope Francis, and is rumored to have been one of the cardinals pushing for Bergoglio's election.
Lehmann was given a red hat by Pope John Paul II in 2001. He retired from his role as head of the Mainz diocese in 2016, and died the morning of March 11, 2018.